PURPOSE: To decrease the number of areas of temperature measruement for the piping by providing the thermocouple to the piping for the liquid sodium and then detecting the leak of the liquid sodium through detection of the temperature difference of the piping.

CONSTITUTION: The closed circuit is formed for the fast reactor for liquid metal cooling by addhering two types of hetero-metal to the lower sides 5 and 6 of piping 1 through which liquid sodium 2 passes, along with installation of thermocouple 4 to be connected to potential difference meter 8 and alarm circuit 9 each. In case some crack is caused at the area near thermocouple contact 5 and 6 of piping 1 and thus the high-temperature liquid sodium leaks there, meter 8 detects the potential difference caused by the temperature difference between contacts 5 and 6 to then give the alarm. In such way, the detection is possible for the leak sodium at two areas via one unit of the detector, thus decreasing the number of the measuring points.